That threat is ridiculus! IF he is underweight she needs to see a lactation specialist to figure out what's wrong with breastfeeding.
Have her get a baby scale, so she can see that he's growing, they recommend a half an ounce a day, so about a lb. a month for a healthy child. So she can monitor his growth at home and not worry about it.
I had something similar happen to me (though no one freaked out about it like this). At 8 weeks my daughter was 10 oz. less then her birthweight because somehow breastfeeding wasn't working out. Unfortunately I lived in KY, a very backward state when it comes to breastfeeding (maybe 1% of mom's do it), and no one could help me. I was also advised to stop breastfeeding and pump and supplement. This was BAD advise. DO NOT STOP BREASTFEEDING. If there is something going on with her milk supply, then this will only make it worse. She should pump in between feedings, and feed that to the baby AFTER the baby has finished at the breast, both sides and seems satisfied. That's my advise, but have her get into contact with La Letche League and or a lactation consultant immediately! This threat is unreasonable and it doesn't sound like its grounded in facts. That doesn't seem like a bad weight, expecially as some babies are small.
Another option, though I wouldn't recommend this normally, is to start rice cereal. They can have it at 4 months, but not other solids, those should wait until 6 months. This will give the baby some extra weight.
If she's having trouble pumping, try some supplements of formula, but be sure to do those AFTER breastfeeding.
Some ways to increase milk supply are the herb fenugreek, mothers milk tea, and a medicine called reglan. I tried the reglan in my situation, BUT it made me really anxious and I had to quit. At that point my daughter REFUSED to latch on and I had no one to help me, so I quit breastfeeding and have regretted it ever since.
Tell her not to freak out but find out some facts! Best wishes!